Add README.md to SearchPage

src/containers/SearchPage/README.md

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+# SearchPage
+
+SearchPage component has roughly 3 sections inside its layout:
+Topbar, MainPanel (for results and filters), and Map
+
+So, rough JSX presentation is something like:
+```jsx
+<Page>
+  <TopbarContainer />
+  <MainPanel />
+  <SearchMap />
+<Page>
+```
+
+Searches can be made by each of these components.
+
+* Topbar contains location search (LocationAutocompleteInput)
+* MainPanel has Filters that can fine-tune current location search
+* SearchMap can create new location searches when the map's bounding box changes
+  (i.e. moving, zooming, etc.)
+
+## Topbar
+
+In contrast to other pages, Topbar gets `currentSearchParams` among other props. This makes it
+possible for Topbar to take current filters into account.
+
+## MainPanel
+
+MainPanel has two functions: showing searchResults and showing filters. There are two sets of
+filters that can be passed to it: `primaryFilters`, `secondaryFilters`.
+
+Primary filters are filters that are shown always on top of SearchResults as dropdown-selections.
+We recommend that only 1 - 3 primary filters are passed in since they start to take too much space
+on narrow screens.
+
+Secondary filters create one more button to the space containing primary filters: *More filters*.
+This more-filters button opens up a SearchFiltersPanel component that can be changed to show those
+extra filters passed to it.
+**Note:** Currently, it doesn't contain any filter components by default, even if you pass in some
+filter data. Creating those filter components is part of the customization process.
+
+On the mobile layout, all the filters are shown in separate mobile filters panel.
+
+## SearchMap
+
+SearchMap listens to 'idle' event and SearchPage function `onIndle` can create a new location search if
+SearchMap's bounds have changed enough.
+
+## Other things to consider
+
+### URL Params vs marketplace-custom-config.js
+
+Filter handling needs some mapping between configured extended data. For example,
+*marketplace-custom-config.js* defines all the categories used by the app, but that data needs to be in
+format `pub_category=selectedCategory` when we talk with the API. (We also use this format in URL.)
+
+This mapping is done at the beginning of the file:
+```
+const CATEGORY_URL_PARAM = 'pub_category';
+const AMENITIES_URL_PARAM = 'pub_amenities';
+
+const customURLParamToConfig = {
+  [CATEGORY_URL_PARAM]: 'categories',
+  [AMENITIES_URL_PARAM]: 'amenities',
+};
+```
+
+### SeachPage schema / SEO
+
+Schema is created inside `createSearchResultSchema` in *SearchPage.helpers.js*. It needs listings
+and address to make meaningful JSON-LD presentation for search engines.